The use of electrically self-heated resistors, hot wires, and hot films as thermal anemometer transducers is well known in the prior art. In such devices, a heated resistive element serves as a sensing element, and its physical geometry is used to define its spatial response to impinging airflow. The sensing transducer has a non-zero temperature coefficient of resistance and is maintained at feedback controlled constant resistance. Transducer element pairs are used to determine direction and in some cases they are used to determine both speed and direction.
In a well-executed paired-element directional thermal anemometer, where speed and direction are both differentially read out, advantage is taken of common-mode rejection of unwanted spurious input signals such as those caused by rain, snow, drizzle, fog, salt accretion and the like. Examples of such anemometers are those made in accordance with the teachings of U.S. Pat. Nos. 4,279,147 and 4,794,795.
Earlier directional anemometers such as those taught by U.S. Pat. Nos. 3,352,154, 3,900,819, 4,024,761, and 4,206,638, use single-ended thermal anemometer sensing element pairs where the velocity or speed component is taken as the sum of the element signals with respect to ground, and direction sign sense information is taken as the difference signal between the two elements.
A bounded pipe or conduit enclosed tandem hot wire transducer is disclosed by U.S. Pat. No. 3,677,085 wherein a method for taking electrical differences from transducer side to side is taught.
A somewhat different approach is disclosed by U.S. Pat. No. 3,498,127, wherein an orthogonal set of paired sensing elements is used to drive a cathode ray indicator displaying speed and direction.
An entirely different approach to directional anemometry takes advantage of the wake vortex that is shed from an obstruction or bluff body that is placed in an air stream. When an obstructing strut or bar is placed across a tube, vortices are shed in rapid succession on opposite sides of the obstruction where the vortex frequency is proportional to fluid velocity through the tube. Directivity is determined by the shape of the tube inlet and direction sign sense is determined by which side of the bluff body leads the shed vortex. Customarily, an ultrasonic method is used to detect the shed vortex. Vortex-Shedding meters are briefly described on pages 262-263 in a book entitled "Fluid Mechanics Measurements", edited by Richard J. Goldstein and published in 1983 by Hemisphere Publishing Corporation, New York, ISBN 0-89116-244-5.
Yet another method that has been widely used in directional anemometry takes advantage of the pressure distribution around a circular cylinder, with pairs of opposing pressure ports being used to determine local pressure differences that are a function of cylinder rotation in the flow field. A detailed description of the technique is presented in Chapter V, "Flow Direction Measurements", pages 97-110 in "Aerodynamics Measurements", edited by Robert C. Dean, Jr., Gas Turbine Laboratory, Massachusetts Institute of Technology, published in 1953 by the M.I.T. Press, Cambridge, Mass. Reference is also made to the teachings of U.S. Pat. No. 3,318,153.
Multi-component anemometers are generally used out-of-doors at unattended or isolated locations and are openly exposed to the surrounding environment. In particular, it has been observed that thermal anemometers may accumulate dirt and contamination and their performance can deteriorate unless routine periodic cleaning is employed or naturally occurring rainfall cleanses the anemometer. Often, they are operated where varying amounts of oil fog and unburned hydrocarbon vapors are present, as at airports, offshore drilling and production platforms, near power plants, and near ship and naval vessel exhaust stacks. Regardless of configuration and how thermal element pairs are used, dirt and oil vapor condensate accumulation adversely affects directional anemometer calibration and, in the long term, measured component wind speed will decrease as anemometer sensitivity becomes impaired, while wind direction sensing is little affected. The differential paired-element anemometers, U.S. Pat. Nos. 4,279,147 and 4,794,795, provide a composite wind speed and direction component output and see little if any change in direction sensing precision since dirt and oil accumulation is random, is fairly uniform, and occurs as a common-mode phenomenon. Wind component output is decreased as the elements become coated, while element cosine response is virtually unchanged. The single-ended element pairs, U.S. Pat. Nos. 3,352,154, 3,900,819, 4,024,761, and 4,206,638, will see a more drastic change in sensitivity since their wind speed output is taken as the sum of two sensing element signals for each component.
Historically, the weather services of the World use wind data in polar form (rho-theta) for wind speed and wind direction, and most mechanical and electromechanical wind sets are configured accordingly. World-wide gathering and transmission of wind data are handled in polar form. For thermal anemometer planar wind measurement, as well as vortex shedding and differential pressure anemometers, two Cartesian components are customarily sensed, indicating North-South and East-West wind, or headwind and crosswind. Generally, wind direction, theta, is determined from two anemometer orthogonal components by computing the arc tangent or arc cotangent function, and the magnitude of the wind resultant, rho, is found by taking the square root of the sum of the squares of the orthogonal wind components.
Allowed patent application Ser. No. 07/568,425 is incorporated by reference to further define background art.